The present invention relates to test equipment for the telecommunications industry. More specifically the invention relates to a subscriber loop tester for testing, verifying, and maintaining telephone switching systems having various telecommunications standards implemented using various protocol and line code technologies.
The telecommunications industry is rapidly developing digital telecommunications technologies to replace analog telephone systems that have been in place for many years. Some digital telecommunications technologies are being implemented under standards established for Integrated Services Digital Networks (ISDN). To better understand the complex nature of these emerging telecommunications technologies, the International Standards Organization (ISO) developed a model for rationalizing the definitions of complicated telecommunications systems. The Open System Interconnect (OSI) model fractures a communications system into seven layers as is shown in FIG. 1. Of the seven layers shown, only the first three are of importance to the present invention. Layer 1 is called the Physical Layer and defines the physical interface for the system or network. The physical layer is responsible for sending and receiving information across the network. Layer 2 is called the Data Link Layer and is responsible for sending and receiving error-free data across the network. Tasks such as error detection and correction are performed by this layer. Layer 3 is called the Network Layer and is responsible for controlling the connections between various nodes on the network. The establishment of a connection from one node to another is controlled by this layer. The upper four layers deal with the types of services provided by the system and are implemented in software. Several different standards from the International Telegraph and Telephone Consultive Committee (CCITT) of the United Nations are specifically pertinent to the lower three layers of the OSI model.
Another important consideration for digital telecommunication is the breaking down of the various user access points within the network. The CCITT has defined various reference points for ISDN communications systems using letter designations. FIG. 2 is a simplified subscriber loop 300 in an ISDN telecommunications system showing these reference points. The loop 300 has a line termination (LT) 302, which may be at a central office switch, a repeater, or the like. A transmission line 304 connects the line termination 302 to the customer premise equipment (CPE) 306. This equipment includes the network termination (NT1) 308 and the terminal equipment (TE) 310. The reference point into the network termination NT1 308 is called the "U" reference point and the interface at this reference point is called the "U" interface. The reference point on the TE 310 side of the network terminator NT1 308 is called the "S/T" reference point and the interface at this reference point is called the "S/T" interface. The network termination NT1 308 is essentially a two wire to four wire converter having a two wire Data Circuit Terminating Equipment, DCE, loop jack at the "U" interface and a four wire Data Terminal Equipment, DTE, loop jack at the "S/T" interface. The four wire side of the network termination acts as a passive bus. Up to eight pieces of terminal equipment TE 310, such as telephones, may be connected to the "S/T" interface.
The CCITT has established standards for the "S/T" interface, which have been generally accepted. Unfortunately, this is not the case on the "U" interface side. This has led to multiple types of protocols for ISDN networks. For example, AT&T and Northern Telecomm have both developed individual proprietary AMI (Alternating Mark Inversion) protocols for ISDN networks. The American National Standards Institute (ANSI) developed its own "U" interface physical layer line code protocol, called 2B1Q (2 Binary, 1 Quaternary Modulation) and established it as the standard for the United States. Another "U" interface physical line code protocol, called 4B3T, has been developed in Europe.
Equipment manufacturers produce integrated circuit IC chip sets for implementing the various protocols. The chip sets include a "U" interface transceiver chip implementing a specific protocol and a "NT/ST" controller for implementing the standards as set down by the CCITT. Both the "U" interface to the transceiver chip and the "S/T" interface to the "NT/ST" controller require line interface circuitry for complying with the pertinent line interface standard, clock extraction circuitry. The interfaces also require transformer(s), protection circuitry, and connection to and termination of the line.
Each "U" interface physical line code protocol for the transceiver has its own characteristics. For example, the data rate for AT&T AMI at the "U" interface is 160 Kbits/sec. For 2B1Q, which is a four level code, the data rate is 80 Kbaud/sec with each baud being equivalent to two bits. In addition, each protocol has a unique power spectral density characteristics, AC and DC terminations, and line levels. The characteristic impedance of the AC termination for AMI is 120 ohms, for 2B1Q it's 135 ohms, and for 4B3T it's 150 ohms.
Between the "U" interface transceiver chip and the "NT/ST" controller chip, the data format and bus structure are defined by the chip manufacturer. This makes it difficult to produce a subscriber loop tester that can handle both the characteristics of the various "U" interface physical layer line code protocols and the data format and bus structure of various chip manufacturers. In addition, there are proprietary mixed analog/digital technologies, such as Meridiam Business Service developed by Norther Telecomm (called P-Phone), implemented in telecommunications systems, and analog POTS (Plain Old Telephone Service) telecommunications systems. Each type of system requires loop testing.
In loop testing, the customer premise equipment (CPE) is replaced by the subscriber loop tester. The subscriber loop tester contains circuitry that functions the same as the network termination (NT1), the terminal equipment (TE), or both in an ISDN system. In a POTS or P-Phone system, the subscriber loop tester functions as the phone. The subscriber loop tester transmits outgoing and receives incoming telecommunications signals from the local loop of the telephone switching network. An example of a subscriber loop tester for testing POTS and AT&T AMI telephone switching systems is the CT-100, manufactured and sold by Tektronix, Inc., Beaverton, Oregon, and shown in block diagram form in FIG. 1. For testing an AT&T AMI digital network, the subscriber loop tester 10 has a "U" interface jack 12 for connecting the two wire line from the telephone switching system. Signals coming from the digital telephone switch pass through a "U" interface attenuator 14 and are coupled to an AT&T AMI card 16 containing the AMI transceiver chip and the AC and DC termination circuits. The AMI line coded signals are converted to data format compatible with AT&T's proprietary K-BUS and couples the formatted data on the K-BUS 18 to a K-BUS controller 20. The K-BUS controller 20 passes 2B+D channel data to the "NT/ST" controller 22 over the K-BUS 18. The K-BUS controller 20 extracts maintenance channel information and makes it available to microprocessor 46 via a parallel interface over digital bus 48. Between the K-BUS controller 20 and the telephone switching network, equipment and maintenance information, such as framing information, synchronization status, when equipment is ready for traffic, and the like, is passed back and forth between the switch and the controller 20 via an embedded communications channel.
The "NT/ST" controller 22 translates the K-BUS formatted data to the "S/T" standard and couples the signals through a termination circuit 24 to a four wire NT jack 26. An external TE device, such as a digital phone, may be connected to the NT jack 26 when the subscriber loop tester 10 is being used to simulate the NT1. A four wire TE jack 28 is provided for connecting an external NT1 to the tester 10 when the tester 10 is simulating a digital phone. The tester 10 may also be configured to simulate both the NT1 and the digital phone. Switch 30 is closed providing a electrical path for the output of the digital phone circuits 32 via S attenuator 34 and termination 36. A speaker 38 and microphone 40 are provided for the digital phone circuits 32 as well as the analog POTS circuits 42. A separate loop jack 44 is provided for connecting a POTS telephone switching network to the tester 10. Configuring and control of various parts of the subscriber loop tester 10 is provided by the microprocessor 46 acting as a controller. The digital bus 48 connects the various parts of the tester 10 to the microprocessor 46.
In operation, a user configures the subscriber loop tester 10 for the particular test to be performed. For example, to test a NT1 device, the tester 10 is configured to simulate the NT1. The two wire loop from the telephone switch is connected to the "U" interface 12 and the digital phone is connected to the NT loop jack 26. Telecommunications signals pass back and forth between the digital phone and the telephone switch through the "U" interface attenuator 14, the AMI card 16, the K-BUS 18, the K-BUS controller 20, "NT/ST" controller 22 and termination 24. The tester 10 may also be configured to replace the NT1 and the digital phone. Switch 30 is closed connecting the digital phone circuits 32 into the communications path of the tester 10. Analog voice data generated by the microphone 40 is converted to digital form in the digital phone circuits 32 and coupled to the "NT/ST" controller 22 via the "S" attenuator 34, termination 36 and switch 30. The "NT/ST" controller translates the digital voice data into a data format compatible with the K-BUS 18. The formatted data is coupled through the K-BUS controller 20 to the AMI card 16. The AMI transceiver converts the formatted data to AMI line coded signals. From the AMI card 16 the signals carrying the voice data and the embedded maintenance channel information from the K-BUS controller are coupled through the "U" interface attenuator 14 and "U" interface loop jack to the digital telephone switching network. Incoming telecommunications information travels the same path as the outgoing information but is routed to the speaker 38.
The tester 10 may be further configured for testing analog POTS telephone switching networks. The analog telephone switching network is connected to the loop jack 44. Analog signals to and from the switch pass through the POTS circuitry 42. Speaker 38 and microphone 40 respectively receive and send analog voice information to the POTS circuitry 42.
Subscriber loop testers, such as the CT-100 have been developed to meet the need for testing one type of "U" interface physical layer line code protocol for an ISDN communications system and for testing analog POTS systems. What is needed is a single subscriber loop tester that has the capability of testing many types of "U" interface physical layer line code protocols in ISDN communication systems as well as testing analog POTS systems, and hybrid analog-digital systems like P-Phone using a single loop input jack. In addition, the subscriber loop tester should be capable of testing phone systems implementing new protocol and proprietary standards that are currently under development.